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Digit forces bias sensorimotor transformations underlying control of fingertip position.

Shibata D, Kappers AM, Santello M - Front Hum Neurosci (2014)

Bottom Line: We hypothesized that, when the tangential forces of the digits are produced in opposite directions, matching error (1) would be biased toward the directions of the tangential forces; and (2) would be greater when the remembered relative contact points are matched with negligible digit force production.However, matching error was not dependent on whether the reference and test hand exerted similar or different forces.We propose that the expected sensory consequence of motor commands for tangential forces in opposite directions overrides estimation of fingertip position through haptic sensory feedback.

View Article: PubMed Central - PubMed

Affiliation: Kinesiology Program, School of Nutrition and Health Promotion, Arizona State University Tempe, AZ, USA.

ABSTRACT
Humans are able to modulate digit forces as a function of position despite changes in digit placement that might occur from trial to trial or when changing grip type for object manipulation. Although this phenomenon is likely to rely on sensing the position of the digits relative to each other and the object, the underlying mechanisms remain unclear. To address this question, we asked subjects (n = 30) to match perceived vertical distance between the center of pressure (CoP) of the thumb and index finger pads (dy ) of the right hand ("reference" hand) using the same hand ("test" hand). The digits of reference hand were passively placed collinearly (dy = 0 mm). Subjects were then asked to exert different combinations of normal and tangential digit forces (Fn and Ftan , respectively) using the reference hand and then match the memorized dy using the test hand. The reference hand exerted Ftan of thumb and index finger in either same or opposite direction. We hypothesized that, when the tangential forces of the digits are produced in opposite directions, matching error (1) would be biased toward the directions of the tangential forces; and (2) would be greater when the remembered relative contact points are matched with negligible digit force production. For the test hand, digit forces were either negligible (0.5-1 N, 0 ± 0.25 N; Experiment 1) or the same as those exerted by the reference hand (Experiment 2).Matching error was biased towards the direction of digit tangential forces: thumb CoP was placed higher than the index finger CoP when thumb and index finger Ftan were directed upward and downward, respectively, and vice versa (p < 0.001). However, matching error was not dependent on whether the reference and test hand exerted similar or different forces. We propose that the expected sensory consequence of motor commands for tangential forces in opposite directions overrides estimation of fingertip position through haptic sensory feedback.

No MeSH data available.


Related in: MedlinePlus

Experimental protocol and conditions (Experiment 1). (A) shows the time course of the experimental protocol. In the “passive dy adjustment” phase, the subject’s thumb and index finger were passively placed by an experimenter to a collinear dy (see Figure 1B). Once the desired dy was reached and digit forces matched the desired target forces, recording of reference hand dy started for 5 s while subjects were asked to perceive and memorize the reference hand dy (“perceive and memorize” phase). During the “relax” phase, subjects were asked to relax their reference hand for 10 s, followed by the “match” phase in which they were asked to reproduce the remembered reference hand dy using the (same) test hand within 10 s. The test hand dy was then recorded for 5 s while subjects maintained the digit position and digit forces (“hold” phase). (B) shows the experimental conditions for Experiment 1. The thumb and index finger (filled and open ellipse, respectively) of the reference hand exerted tangential forces either in the same or opposite directions (“Same” and “Opposite”, left and middle column, respectively). In the Same condition, thumb and index finger exerted tangential forces that were both upward or downward (TUP-IUP or TDOWN-IDOWN, respectively). In the Opposite condition, the tangential forces of the thumb and index finger were directed opposite to each other, i.e., either upward and downward (TUP-IDOWN) or downward and upward (TDOWN-IUP), respectively. In the Control condition (right column), subjects were asked to exert no tangential force while exerting large or negligible normal forces (“Fn only” or “No Ftan/Fn”, respectively). The magnitude of tangential and normal forces was the same across these conditions (Ftan: 2.5–3.5 N, Fn: 4–5 N) with the exception of the “No Ftan/Fn” condition (Ftan: 0 ± 0.25 N, Fn: 0.5–1 N). The test hand in Experiment 1 exerted only negligible tangential and normal forces (Ftan: 0 ± 0.25 N, Fn: 0.5–1 N).
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Figure 2: Experimental protocol and conditions (Experiment 1). (A) shows the time course of the experimental protocol. In the “passive dy adjustment” phase, the subject’s thumb and index finger were passively placed by an experimenter to a collinear dy (see Figure 1B). Once the desired dy was reached and digit forces matched the desired target forces, recording of reference hand dy started for 5 s while subjects were asked to perceive and memorize the reference hand dy (“perceive and memorize” phase). During the “relax” phase, subjects were asked to relax their reference hand for 10 s, followed by the “match” phase in which they were asked to reproduce the remembered reference hand dy using the (same) test hand within 10 s. The test hand dy was then recorded for 5 s while subjects maintained the digit position and digit forces (“hold” phase). (B) shows the experimental conditions for Experiment 1. The thumb and index finger (filled and open ellipse, respectively) of the reference hand exerted tangential forces either in the same or opposite directions (“Same” and “Opposite”, left and middle column, respectively). In the Same condition, thumb and index finger exerted tangential forces that were both upward or downward (TUP-IUP or TDOWN-IDOWN, respectively). In the Opposite condition, the tangential forces of the thumb and index finger were directed opposite to each other, i.e., either upward and downward (TUP-IDOWN) or downward and upward (TDOWN-IUP), respectively. In the Control condition (right column), subjects were asked to exert no tangential force while exerting large or negligible normal forces (“Fn only” or “No Ftan/Fn”, respectively). The magnitude of tangential and normal forces was the same across these conditions (Ftan: 2.5–3.5 N, Fn: 4–5 N) with the exception of the “No Ftan/Fn” condition (Ftan: 0 ± 0.25 N, Fn: 0.5–1 N). The test hand in Experiment 1 exerted only negligible tangential and normal forces (Ftan: 0 ± 0.25 N, Fn: 0.5–1 N).

Mentions: For both experiments, after subjects’ digits were passively moved (“passive dy adjustment” phase, Figure 2A), we asked subjects to perceive and memorize the vertical distance (dy) between thumb and index CoP of the right hand (“reference” hand) (“perceive and memorize” phase, Figure 2A), relax for 10 s, and match it using the same hand (“test” hand) (“match” phase, Figure 2A). We used a 10-s delay between the “perceive and memorize” and “match” phase in our previous work to ensure memory formation (Shibata et al., 2013). We used the same delay in the present study to allow comparison with our previous work. An important difference between the present study and our previous work (Shibata et al., 2013) is that subjects were asked to exert normal and tangential digit forces with different combinations of magnitude and direction during the “perceive and memorize” phase (see below).


Digit forces bias sensorimotor transformations underlying control of fingertip position.

Shibata D, Kappers AM, Santello M - Front Hum Neurosci (2014)

Experimental protocol and conditions (Experiment 1). (A) shows the time course of the experimental protocol. In the “passive dy adjustment” phase, the subject’s thumb and index finger were passively placed by an experimenter to a collinear dy (see Figure 1B). Once the desired dy was reached and digit forces matched the desired target forces, recording of reference hand dy started for 5 s while subjects were asked to perceive and memorize the reference hand dy (“perceive and memorize” phase). During the “relax” phase, subjects were asked to relax their reference hand for 10 s, followed by the “match” phase in which they were asked to reproduce the remembered reference hand dy using the (same) test hand within 10 s. The test hand dy was then recorded for 5 s while subjects maintained the digit position and digit forces (“hold” phase). (B) shows the experimental conditions for Experiment 1. The thumb and index finger (filled and open ellipse, respectively) of the reference hand exerted tangential forces either in the same or opposite directions (“Same” and “Opposite”, left and middle column, respectively). In the Same condition, thumb and index finger exerted tangential forces that were both upward or downward (TUP-IUP or TDOWN-IDOWN, respectively). In the Opposite condition, the tangential forces of the thumb and index finger were directed opposite to each other, i.e., either upward and downward (TUP-IDOWN) or downward and upward (TDOWN-IUP), respectively. In the Control condition (right column), subjects were asked to exert no tangential force while exerting large or negligible normal forces (“Fn only” or “No Ftan/Fn”, respectively). The magnitude of tangential and normal forces was the same across these conditions (Ftan: 2.5–3.5 N, Fn: 4–5 N) with the exception of the “No Ftan/Fn” condition (Ftan: 0 ± 0.25 N, Fn: 0.5–1 N). The test hand in Experiment 1 exerted only negligible tangential and normal forces (Ftan: 0 ± 0.25 N, Fn: 0.5–1 N).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4120687&req=5

Figure 2: Experimental protocol and conditions (Experiment 1). (A) shows the time course of the experimental protocol. In the “passive dy adjustment” phase, the subject’s thumb and index finger were passively placed by an experimenter to a collinear dy (see Figure 1B). Once the desired dy was reached and digit forces matched the desired target forces, recording of reference hand dy started for 5 s while subjects were asked to perceive and memorize the reference hand dy (“perceive and memorize” phase). During the “relax” phase, subjects were asked to relax their reference hand for 10 s, followed by the “match” phase in which they were asked to reproduce the remembered reference hand dy using the (same) test hand within 10 s. The test hand dy was then recorded for 5 s while subjects maintained the digit position and digit forces (“hold” phase). (B) shows the experimental conditions for Experiment 1. The thumb and index finger (filled and open ellipse, respectively) of the reference hand exerted tangential forces either in the same or opposite directions (“Same” and “Opposite”, left and middle column, respectively). In the Same condition, thumb and index finger exerted tangential forces that were both upward or downward (TUP-IUP or TDOWN-IDOWN, respectively). In the Opposite condition, the tangential forces of the thumb and index finger were directed opposite to each other, i.e., either upward and downward (TUP-IDOWN) or downward and upward (TDOWN-IUP), respectively. In the Control condition (right column), subjects were asked to exert no tangential force while exerting large or negligible normal forces (“Fn only” or “No Ftan/Fn”, respectively). The magnitude of tangential and normal forces was the same across these conditions (Ftan: 2.5–3.5 N, Fn: 4–5 N) with the exception of the “No Ftan/Fn” condition (Ftan: 0 ± 0.25 N, Fn: 0.5–1 N). The test hand in Experiment 1 exerted only negligible tangential and normal forces (Ftan: 0 ± 0.25 N, Fn: 0.5–1 N).
Mentions: For both experiments, after subjects’ digits were passively moved (“passive dy adjustment” phase, Figure 2A), we asked subjects to perceive and memorize the vertical distance (dy) between thumb and index CoP of the right hand (“reference” hand) (“perceive and memorize” phase, Figure 2A), relax for 10 s, and match it using the same hand (“test” hand) (“match” phase, Figure 2A). We used a 10-s delay between the “perceive and memorize” and “match” phase in our previous work to ensure memory formation (Shibata et al., 2013). We used the same delay in the present study to allow comparison with our previous work. An important difference between the present study and our previous work (Shibata et al., 2013) is that subjects were asked to exert normal and tangential digit forces with different combinations of magnitude and direction during the “perceive and memorize” phase (see below).

Bottom Line: We hypothesized that, when the tangential forces of the digits are produced in opposite directions, matching error (1) would be biased toward the directions of the tangential forces; and (2) would be greater when the remembered relative contact points are matched with negligible digit force production.However, matching error was not dependent on whether the reference and test hand exerted similar or different forces.We propose that the expected sensory consequence of motor commands for tangential forces in opposite directions overrides estimation of fingertip position through haptic sensory feedback.

View Article: PubMed Central - PubMed

Affiliation: Kinesiology Program, School of Nutrition and Health Promotion, Arizona State University Tempe, AZ, USA.

ABSTRACT
Humans are able to modulate digit forces as a function of position despite changes in digit placement that might occur from trial to trial or when changing grip type for object manipulation. Although this phenomenon is likely to rely on sensing the position of the digits relative to each other and the object, the underlying mechanisms remain unclear. To address this question, we asked subjects (n = 30) to match perceived vertical distance between the center of pressure (CoP) of the thumb and index finger pads (dy ) of the right hand ("reference" hand) using the same hand ("test" hand). The digits of reference hand were passively placed collinearly (dy = 0 mm). Subjects were then asked to exert different combinations of normal and tangential digit forces (Fn and Ftan , respectively) using the reference hand and then match the memorized dy using the test hand. The reference hand exerted Ftan of thumb and index finger in either same or opposite direction. We hypothesized that, when the tangential forces of the digits are produced in opposite directions, matching error (1) would be biased toward the directions of the tangential forces; and (2) would be greater when the remembered relative contact points are matched with negligible digit force production. For the test hand, digit forces were either negligible (0.5-1 N, 0 ± 0.25 N; Experiment 1) or the same as those exerted by the reference hand (Experiment 2).Matching error was biased towards the direction of digit tangential forces: thumb CoP was placed higher than the index finger CoP when thumb and index finger Ftan were directed upward and downward, respectively, and vice versa (p < 0.001). However, matching error was not dependent on whether the reference and test hand exerted similar or different forces. We propose that the expected sensory consequence of motor commands for tangential forces in opposite directions overrides estimation of fingertip position through haptic sensory feedback.

No MeSH data available.


Related in: MedlinePlus